1. Field of the Invention
This invention relates to metal-insulator-metal tunneling devices/diodes (MIM TDs) and method of making such devices, and more specifically, to the design of tunneling junctions in a non-planar configuration designed for room temperature operation at sub millimeter, infrared, and optical frequencies.
2. Description of the Prior Art
A MIM TD is of significant interest because of its high speed of response. Its operation is based on the well known effect that if two metal electrodes are separated by a very thin insulating film or vacuum gap, current can flow between the two metal electrodes via tunneling [1]. In addition to the magnitude of asymmetry in the I-V characteristic, which defines rectification efficiency of the MIM TD, operation at the infrared and optical frequency region requires minimal parasitic shunt capacitance, series resistance, skin effect, thermal resistance and junction area. Moreover, the diode must provide stable characteristics and the structural configuration should be suitable for use in monolithic integrated circuits and arrays [2]. Because of the MIM TD's fast switching properties, it can be used in very high frequency applications. In fact, whisker-type or point-contact MIM diodes hold the current record of operation at 150 THz [3,4]. Whisker-type MIM TD structures have been successfully used as detectors and mixers at frequencies ranging from sub-millimeter to infrared wavelengths with reasonably good performance [5]. FIG. 1 shows a whisker MIM TD. It consists of metal substrate 101, tunneling oxide layer 102, and metal whisker 103. The major disadvantages of the whisker-type structure are their low mechanical stability as well as its incompatibility with integrated circuit system technology. Therefore, significant efforts have been made to integrate MIM TDs in planar configurations as those shown in FIG. 2. In the device shown in FIG. 2 [6], a thin oxide layer 102 sandwiched between two crossing metal planar electrodes, namely metal strip 101 and metal strip 103 deposited on a dielectric substrate 100, creates a potential barrier that electrons can tunnel. However, this design exhibits a number of disadvantages such as high series resistance due to thin metal layers, high parasitic shunt capacitance, large junction areas in the order of 10−7 cm2 to 10−8 cm2 because of the lithography limitations, and an undesirably high skin resistance. All of these disadvantages limit the operating efficiency of these devices at frequencies in the order of 1 THz or higher. To overcome the disadvantages of the MIM TD designs described above, Calviello [2] proposed a quasi-planar MIM TD configuration shown in FIG. 3. In this prior art design configuration, the MIM tunneling junction 110 is formed in the area between peninsula-like projection of the metal layer 101 deposited on a dielectric substrate 100 and metal layer 103 through the thin dielectric layer 102. Here, metallization layer 104 made of 50-100 nm thick gold is used to reduce temperature dependence of the MIM TD structure. The tunneling device of the configuration shown in FIG. 3 is claimed to have a junction area in the 10−10 to 10−11 cm2 range, which indicates the feasibility of the detection in 10 micron wavelength range [2]. This design still exhibits a number of limitations that include technical difficulties in low temperature fabrication of layer 105 (FIG. 3) having low dielectric loss and high thickness as well as still relatively high resistance due to extremely thin 101 metal layer in addition to difficulties in formation of an open and smooth edge of 101-1 electrode with a pinhole-free ultrathin tunneling dielectric layer 102, which are neighboring with a thick dielectric layer 105.
Another prior art processing technique of ultra-small tunneling junction fabrication is described by Ootuka et. al [7]. In this approach, silicon nitride membrane having small windows is used as a shadow mask to vacuum-deposit a metal film of fine structure onto a substrate. By means of double-angled evaporation, which consists in the deposition of a metal film with the same shape as the mask, oxidation of the metal layer, and finally, the deposition of the second metal layer of the same shape at a different angle, small tunnel-junctions are formed by overlapping the first and second metals through the thin dielectric layer. This method has important advantages over traditional photolithography processing, that is, the lift-off processes after vacuum deposition is not needed, and the plasma oxidation can be used for making tunneling barriers. Several limits of this method are: (1) there is a limitation in mask-alignment position setting; (2) there is broadening of the pattern during deposition mainly because of the finite gap between a mask and a substrate; and (3) a mask with large area is difficult to make because of the weakness of the membrane. The other traditional shadow-mask processes have suffered from a lack of robustness at dimensions below 75 nm, with significant run-to-run variability and poor process control. Current shadow-mask processes typically use a resist bilayer consisting of a thin poly-methylmethacrylate (PMMA) imaging layer on top of a thicker support layer of low-molecular-weight PMMA or PMMA/methacrylic acid copolymer, relying on the difference in sensitivity between the two resists to produce suitable undercut [8]. However, the long development time needed to create large undercut regions in the support layer can cause significant biasing of the imaging layer features, degrading process resolution which results in significant run-to-run variability and poor process control for the formation of the features below 75 nm.
There are various applications of MIM TDs. Particularly, they can be used for sensing terahertz/infrared/optical radiation using nonlinear characteristics of a tunneling device. Also, it can be used for the direct conversion of infrared and even visible light energy into electrical power by direct rectification of ultrahigh frequency electromagnetic wave. In fact, for conventional photovoltaic technologies, the solar energy conversion efficiency is fundamentally limited to approximately 30% because the photon energy that is significantly above the bandgap is lost (up to 24%). In contrast, an adequately designed antenna array in conjunction with an ideal non-linear MIM tunneling diode can efficiently absorb almost the entire solar spectrum, with the solar spectrum conversion efficiencies more than 85% [9]. Antenna-coupled infrared and optical detector consists of an antenna and a rectifier. It operates in the same way as a well known radio antenna at lower frequencies. In this detector, the electric field from an incident electromagnetic radiation source will induce a wave of accelerated electric charge in a conductor. Efficient collection of incident radiation is then dependent on resonance length scales and impedance matching of the antenna to the rectifier to prevent losses. In addition to a lot of attempts to use MIM tunneling junctions for detection of optical and infrared radiation [10], the non-linear electronic properties of MIM tunneling junctions is a great deal of practical interest to the spontaneous light emission from non-equilibrium MIM tunneling junction cause by photon-assisted tunneling of electrons [11]. The most important features of photon-assisted electron tunneling are the possibility of modulating the light frequency by varying the operating voltage [12].
What is needed in the art are simplified designs and methods of fabricating MIM TDs that the ability to produce a very small junction area MIM TD that are free from the described above limitations. The present invention satisfies those needs.